<?php if ( (time() - 24*60*60) < strtotime($topic->topic_time) ) : ?>*NEW* <?php endif; ?>
A very rough way of doing it there, but if you put that in
forum.php before say,
<?php topic_title(); ?> if you want it in the topic title, it should work. Change
24*60*60 to the number of seconds you want to include topics from.